In petroleum processing, aromatic streams are derived from processes such as naphtha reforming and thermal cracking (pyrolysis). These aromatic streams also contain undesirable hydrocarbon contaminants including mono-olefins, dienes, styrenes and heavy aromatic compounds such as anthracenes.
The aromatic streams are used as feedstocks in various subsequent petrochemical processes. In certain of these processes, such as para-xylene production, e.g., from an aromatic stream containing benzene, toluene and xylene (BTX) or toluene disproportionation, hydrocarbon contaminants cause undesirable side reactions. Therefore the hydrocarbon contaminants must be removed before subsequent processing of the aromatic streams.
Moreover, the shift from high-pressure semiregenerative reformers to low-pressure moving bed reformers results in a substantial increase in contaminants in the reformate derived streams. This in turn results in a greater need for more efficient and less expensive methods for removal of hydrocarbon contaminants from the aromatic streams.
Undesirable hydrocarbon contaminants containing olefinic bonds are quantified by the Bromine Index (BI). Undesirable olefins, including both dienes and mono-olefins, have typically been concurrently removed from aromatic streams such as BTX by contacting the aromatic stream with acid-treated clay. Other materials, e.g., zeolites, have also been used for this purpose. Clay is an amorphous naturally-occurring material, while zeolites used for this purpose generally are synthesized and are therefore more expensive. Both clay and zeolites have very limited lifetimes in aromatics treatment services. The length of service correlates with the level of bromine reactive impurities (“BI-reactive” impurities or contaminants) in the feedstream. BI-reactive contaminants rapidly age both clay and zeolites. Indeed, although clay is the less expensive of the two alternatives, large aromatic plants can spend more than a million dollars a year on clay. Furthermore, since zeolites are considerably more expensive than clay, their use in removing hydrocarbon contaminants can only be justified by dramatically improved stability in aromatics treatment so that their cycle length is practical.
U.S. Pat. Nos. 6,368,496 and 6,781,023 teach bromine reactive hydrocarbon contaminants are removed from aromatic streams by first providing an aromatic feedstream having a negligible diene level. The feedstream is contacted with an acid active zeolite catalyst composition under conditions sufficient to remove mono-olefins. The aromatic stream may be pretreated to remove dienes by contacting the stream with clay, hydrogenation or hydrotreating catalyst under conditions sufficient to substantially remove dienes but not monolefins.
U.S. Pat. No. 7,517,824 teach a method of removing hydrocarbon feed residue and regeneration of a catalyst used to reduce BI, but fails to recognize that leaving some coke deposition on the catalyst improves results.
Other relevant references include, U.S. Pat. Nos. 6,500,996; 6,781,023 and U.S. patent application Publications.
Removing trace olefins from the product of refinery naphtha reformers with zeolites is a relatively new commercial process. In the process it is meant to replace, the trace olefins and other contaminants are removed from reformate using acid treated clay. The clay acts as much like a sorbent as like a catalyst. The clay stays on stream for several weeks to many months depending upon the concentration of contaminants. During this time, carbonaceous deposits build up to as much as 40 wt % of the spent “clay” removed from the reactor. The clay and absorbed contaminants are removed from the reactor and sent to landfill. Clay regeneration is not economically practical.
Zeolites used in refining and petrochemical processes are often, but not always, regenerated. Regeneration of zeolites is only practiced if the economics of regeneration are better than the economics of using fresh catalyst. Part of the many considerations include the extent to which burning off the carbon can restore most of the activity. Generally it was heretofore believed that burning off carbon does not restore activity if other contaminants such as metals, steam, or halides have caused irreversible loss of acid sites.
Furthermore, even if the zeolite can be regenerated, it is not obvious that the regeneration can be accomplished at an attractive cost. Regeneration cost is a function of coke on catalyst and regeneration conditions. Spent catalyst with 30 to 40 wt % coke is roughly 5 times more expensive to regenerate than typical spent zeolite catalysts containing closer to 10 wt % coke. Zeolite catalyst used in clay treating deactivates steadily over the course of the cycle as coke deposits on the catalyst and blocks access to the catalytic sites responsible for contaminant removal. The coke on spent MCM-22 catalysts removed from clay treating reactors is typically between 7.5 and 15 wt %. This is surprisingly lower than the 30 to 50 wt % coke typically found on clay removed from aromatics feedstock pretreaters.
There is still a need for an improved method to replace clay treating for the removal of trace olefins from reformate.
The present inventors have surprisingly discovered an improved process of regenerating zeolite catalysts for use in removal of contaminants in an aromatics stream that has the advantages, in embodiments, of the use of milder conditions, while at the same time improving the activity of the regenerated catalyst.